fix: update item_tax_rate in backend

[ljain112]

Issue: item_tax_rate is only set from the front end.
If the Item tax template is updated from the backend incorrect taxes are calculated.

Steps on how to replicate this issue

• Create one item with only one item tax template
• Create a transaction for this item. This should auto-fetch item tax template.
• Unset item tax template from the item. Save

With unset, it doesn't unset item tax rate, causing incorrect final taxes.

Closes: #41081
Frappe Support Issue: https://support.frappe.io/app/hd-ticket/27836

backport-version-15-hotfix
backport-varsion-14-hotfix

[ruthra-kumar]

@ljain112

#44293 already moved the tax applying logic to server-side on develop branch. Better to make this change directly on v15

[ljain112]

@ljain112

#44293 already moved the tax applying logic to server-side on develop branch. Better to make this change directly on v15

@ruthra-kumar
The issue is still replicable in develop branch after enabling serve_side_reactivity because item_tax_template is updated in calculate_taxes_and_charges.